1. Field of Invention
The present invention relates to liquid crystal devices, projection display devices, and manufacturing methods for substrates for liquid crystal devices. This invention particularly relates to the construction of desirable liquid crystal devices to be used as a light source for a liquid crystal projector, and to a manufacturing method for a substrate for a liquid crystal device.
2. Description of Related Art
For a projection liquid crystal display device, such as a liquid crystal projector, there is the three-panel type in which three liquid crystal panels corresponding to the three primary colors red (R), green (G), and blue (B) are used, and the one-panel type in which one liquid crystal panel and a color generating device are used. For a liquid crystal light source which is a part of such a projection liquid crystal display device, active matrix type liquid crystal panels are typically used.
Also, a liquid crystal panel comprises, for example an active matrix type liquid crystal light source and polarizing plates which are disposed in the front and in the rear of the active matrix type liquid crystal light source. FIG. 18 is a cross section showing an example of the construction of such a conventional liquid crystal light source.
A liquid crystal light source is made such that liquid crystal is enclosed between two transparent substrates as shown in FIG. 18. The liquid crystal light valve is provided with a thin film transistor (hereinafter called TFTs) array substrate 10 and facing substrate 20 which is disposed to face the TFT array substrate.
On the TFT array substrate 10, a plurality of scanning lines 3a and a plurality of data line 6a are disposed so as to cross like in a lattice. A pixel switching TFT 30 is disposed corresponding to the cross points of the scanning lines 3a and the data lines 6a. The scanning lines 3a cross over a semiconductor layer 1a of the TFT 30 via insulating thin layer 2, and channel area 1a′ is formed in a crossing area of the semiconductor layer 1a. A data line 6a crossing over the scanning line 3a is connected electrically to a source area 1d of the semiconductor layer 1a via contact hole 5. Also, pixel electrode 9a is formed in an area which is surrounded by the scanning lines 3a and the data lines 6a on an upper layer of the data line 6a. The pixel electrode 9a is connected electrically to a drain area 1e of the semiconductor layer 1a via the contact hole 8. An alignment layer 16 subjected to an alignment layer treatment by a rubbing treatment is formed on the pixel electrode 9a. The alignment layer 16 is formed by an organic layer of polyimide.
In such a TFT array substrate 10, as compared to the area on which the pixel electrode 9a is formed, the thickness of the area in which TFT 30 is a non-pixel area, the thickness of the area in which the scanning line 3a is formed, and the thickness of the area in which data line 6a is formed tends to be large because the insulating layers 4 and 7 for such areas and the wiring are layered therein; thus, the gap section is formed on the surface of the alignment layer 16. The gap is particularly large between the area where the TFT 30 is formed and the area where the pixel electrode 9a is formed. Furthermore, if a first shading layer 11a is formed under TFT 30 so as to shield a capacity line 3b and TFT 30 for higher quality display, the gap section tends to be reality visible.
Recently, more finely pitched pixels tend to be desired according to the requirements for size reduction of the liquid crystal light source in view of greater fineness and greater brightness of the liquid crystal projector. However, for example, if the pixel pitch becomes as fine as 20 μm, there will be areas where effective rubbing treatment on the alignment layer is impossible because of the gap section on the underlayer of the alignment layer in the case of liquid crystal light valve in which an alignment layer made of an organic layer such as polyimide is provided; thus, disclination of the liquid crystal occurs nearby sometimes degrading display quality. Such a problem becomes more apparent if the pixel pitch is made finer.
Also, the intensity of the light incident on the light valve has increased for brighter liquid crystal projectors. Because of this, the alignment layer made of an organic layer such as of polyimide deteriorates due to light and heat, and alignment uniformity of the alignment layer decreases. Thus, the orientation of the liquid crystal molecules lose uniformity, the contrast of the display decreases, and sometimes ultimately leads to inferior display quality. The reasons such problems occur is that the organic layer made of polyimide absorbs some amount of the 400 to 450 nm wavelength visible light, the alignment layer deteriorates due to the absorption of the light, the orientations of the liquid crystal lose alignment uniformity near the deteriorated area of the alignment layer, and thus degraded display quality results.
In order to solve such problems, a light source has been provided in which the alignment layer is made of a layer obtained by oblique evaporation of inorganic material such silicon oxide (SiO) instead of an organic layer such as polyimide, and in such a way that the liquid crystal molecules are oriented unidirectionally by the surface forming effect of the inorganic oblique evaporation layer. The alignment layer made of an inorganic oblique evaporation layer can be formed by unidirectionally vacuum-evaporating the inorganic material onto a substrate fixed at a certain angle, more specifically from a direction slanted by 10 to 30 degrees to the substrate so as to grow the columnar structure of the inorganic material disposed at a predetermined angle to the substrate, and such a method is called a oblique evaporation method. The alignment layer obtained in this way has advantages such as superior light resistance and heat-resistance as compared to the alignment layer made of an organic material such as polyimide due to its inorganic layer construction, superior durability of the liquid crystal light valve, and loss of alignment uniformity of the liquid crystal caused by incorrect rubbing treatment seen in the case of the organic layer such as one of polyimide, even if the pixel pitch is made finer.
However in contrast to advantages such as light resistance and heat resistance, an alignment layer made of an inorganic layer has disadvantages such as weak alignment uniformity of liquid crystals as compared to an alignment layer made of an organic layer. Accordingly, in a liquid crystal device using an inorganic alignment layer, disclination easily occurs if any factor occurs causing loss of alignment uniformity of the liquid crystals; inferior display is provided. Specifically, surfaces of the active matrix substrate forming the liquid crystal light valve become irregular when forming switching elements such as TFTs, signal lines such as data lines and scanning lines, and pixel electrodes on the active matrix substrate. Such irregularities in the surfaces produce shadows on the substrate during oblique evaporation; thus, defective parts may sometimes be generated on the alignment layer. In the case in which there are such defects on the alignment layer, the organic layer may be able to compensate for the defect by its own sufficient aligning uniformity. However, the aligning uniformity of the inorganic evaporation layer is so weak that disclination may be caused. Because of this tendency, inferior display such as one in which there is light leakage in the domain in which the alignment direction is different occurs, and the display quality decreases due to low contrast.
As a solution for reducing the disclination, there is a method called a pre-tilt method in which the liquid crystal molecules are disposed slant to the surface of the substrate in advance when no voltage is impressed. Generally, the disclination can be progressively reduced as the pre-tilt angle increases. However, if the pre-tilt angle is increased in the case of an inorganic alignment layer in which the aligning uniformity was originally weak, the spiral structure of the liquid crystals between the substrates becomes unstable. Therefore, inferior display is produced due to the existence of reverse twist domains which are areas in which the twisting directions of liquid crystals becomes partially opposite.
This problem also occurs in liquid crystal devices using alignment layers made of an inorganic oblique evaporation layer formed on the underlayer on which surface the gap section exists.
Above problem is not limited to the case of an active matrix type liquid crystal device using a 3-terminal-type-element such as a TFT element; but it is a common problem among active matrix type liquid crystal devices using 2-terminal-type-elements such as a Thin-Film-Diode (hereinafter called TFD) and passive matrix type liquid crystal devices whenever an inorganic alignment layer is used in the liquid crystal device.